How To Determine Well Pump Flow Rate?

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Well Water Pressure Tank

Determining your well pump flow rate is crucial to ensure that your well filtration system is operating at peak efficiency. Backwashing iron filters, water softeners, acid neutralizers, and miscellaneous well water filtration systems come in different sizes. Matching the wrong size with your well flow rate will lead to disappointing results. But how can you find out what your well pump flow rate is?

Your well water flow rate refers to the gallons of water your well pump can deliver per minute or GPM. Using only a 5-gallon bucket and a stopwatch or timer, you can get an approximation of your flow rate. Our step-by-step instructions below will walk you through exactly how to calculate it.

Quick Summary

  • Determining your pump flow rate is done by timing the well pump cycle and counting the gallons of water drawn.
  • The formula to calculate the well pump flow rate is: GPM = (gallons drawn) / (number of seconds timed for the pump cycle) x 60

Why Knowing Your Well Water Flow Rate Is Important

If you have a well, you need to understand your well’s water flow rate. The well water flow rate refers to the amount of water your well pump can deliver per minute or hour.  Knowing this rate is essential for the following reasons:

  • Selecting The Proper Well Water Treatment System: Measuring the flow rate of a well pump is crucial because many well water treatment systems, including iron filters, require twice the backwash flow rate as the service rate. It is important to have a good flow rate to ensure the filter systems are appropriately back-washed and to provide adequate water pressure to the home.
  • Better Understanding Of Available Water For Daily Use: Second, it lets you understand how much water you have available daily. This is particularly important for households with high water usage needs, such as those with large families or homes that use water for irrigation or livestock.
  • Make Informed Decisions About Water Usage: Third, knowing your well water flow rate will enable you to decide about water usage. If your well delivers water slower than you expected, you may need to adjust your water usage habits or consider upgrading your pump or well system.
  • Correctly Identify And Troubleshoot Issues With The Well Pump: Finally, knowing your well water flow rate is essential for troubleshooting any issues with your well pump. If you notice a sudden drop in water pressure or a change in water quality, understanding your well water flow rate can help you identify the root cause of the problem and take appropriate action.

What Exactly Is Flow Rate

How To Determine Well Pump Flow Rate

Flow rate refers to the amount of water that can be delivered by a well pump in a given period. It is typically measured in gallons per minute (GPM) or per hour (GPH).

The formula involves dividing the gallons drawn by the number of seconds timed for the pump cycle and multiplying by 60 to get the pump’s gallons per minute (GPM).

GPM = (Gallons drawn) / (Number of seconds timed for the pump cycle) x 60

For example, if your well pump can fill a 5-gallon bucket in one minute directly from your well, the flow rate is 5 GPM.

Several factors can impact the flow rate, including the well’s size and depth, the pump’s capacity, and the rate at which water is extracted from the well. Factors such as well diameter, screen length, and obstructions or sediment can also affect the flow rate.

Pro Tip: Simply timing the water dispensed from a spigot near the well will not provide accurate results. Not only does the water need to travel to the spigot, but also the hose bib will typically restrict the true water flow rate. Therefore, calculating flow rate from the well pump will provide the most accurate results.

Submersible Pump Vs Constant Pressure Systems

Most private wells are equipped with a pressure tank and single-speed submersible pump. These pressure tank systems can be identified because they generally use a large blue tank. The pump on a standard submersible pressure tank will turn on at around 30 PSI and turns back off when the pressure reaches around 60 PSI.

On the other hand, if you have a constant pressure system, this flow rate calculation will not work. The pressure tanks on a constant system are much smaller and are designed to keep the same water pressure at all times. If you have this type of system, please contact us to find the correct flow rate.

How To Calculate Well Pump Flow Rate Step-By-Step

1. Ensure Well Pump Is At Full Pressure

Make sure the well pump is at full pressure by running water in the house or from a hose bib until you hear the pump kick in or the pressure switch click.

2. Close All Faucets

Close all faucets and make sure no water is being used. The well pump will build pressure for a few minutes, and the pressure switch will turn off.

3. Open Hose Bib

Open a hose bib located near or after the pressure tank.

4. Fill 5-Gallon Bucket

Fill a 5-gallon bucket with water from the hose bib, keeping track of the water collected. If the bucket gets full, empty it and continue measuring.

5. Empty Pressure Tank

Measure the number of gallons drawn from the pressure tank until the pump turns on again.

6. Close Hose Bib

When you hear the pump turn on, close the hose bib.

7. Measure Time To Build Pressure

Time the interval it takes for the well pump to build back pressure, which is the time between the cut-in and cut-out of the pressure switch. The switch turns on the pump at a lower pressure and turns it off when it reaches a higher pressure.

8. Calculate Flow Rate

Use the following formula to determine the flow rate: GPM = (Gallons drawn) / (Number of seconds timed for the pump cycle) x 60

Let’s say you draw down 10 gallons, and it takes 50 seconds to build back up to pressure.
To calculate the flow rate, you would divide the gallons drawn down by the time it takes for the pump to recover and then multiply the result by 60. So, in this case, the calculation would be:

GPM = (10 gallons / 50 seconds) = 0.2 gallons per second
Then 0.2 x 60 = 12 gallons per minute (GPM) flow rate

Therefore, the flow rate for this example is 12 GPM.

Matching Your Well Pump and Iron Filter

Matching Well Water Filter To Well Water Pump Flow Rate

Suppose you intend to set up an iron filter that requires a backwash rate of 7 gallons per minute to effectively remove the accumulated iron. However, your well pump can only pump at 6 gallons per minute. Here’s the problem! If your well pump cannot support the required backwash rate, the filter cannot effectively clean the media.

In such a situation, you have several alternatives to choosing the right treatment option:

  • Obtain a smaller iron filter that can perform backwash at a rate of 7 gallons per minute.
  • Upgrade the well pump’s size or output, but the well may not be capable of delivering a higher water output.
  • Install a small storage tank where the water initially flows from the well, and then a booster pump with an appropriate flow rate can be used to increase the pressure after the tank.

Related: Our Favorite Well Water Filters Guide

FAQs For Well Water Flow Rate

What is a typical well flow rate?

The Water Well Board recommends a flow rate of 5 gallons per minute (gpm) for two hours, equivalent to a minimum of 600 gallons of water within a two-hour period once daily. 

On the other hand, the New Hampshire Water Well Association recommends a flow rate of 4 gpm for four hours, equivalent to a total of 960 gallons of water within a four-hour period. However, depending on the family size and outdoor usage requirements, some homeowners may prefer a higher capacity than these suggested amounts.

How to calculate well pump flow rate from RPM

To calculate flow rate from revolutions per minute (RPM), multiply the RPM by the displacement, then divide by 231.

Flow rate (gallons per minute) = (RPM x displacement) ÷ 231

How do I know what size well pump I need?

Determining the appropriate size of the well pump is a crucial step based on the well’s yield and the household’s requirements. Following a general rule that advises against installing a pump with a greater capacity than the well is essential to avoid potential issues.

Calculating the required size of a water pump depends on various factors, such as the flow rate capacity of the well, the lift required, and friction losses in the piping system.  Vertical rise, horizontal pumping distance, and water quantity usage rate are also considered in the calculation. 

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